Recently, the ion-exchange membrane process is experiencing, for its merit of energy saving, quality product and non-pollution, a reputation that this process is superior to conventional amalgam or diaphragm processes. It is also known that a key factor for successful operation thereof depends on full utilization of the capability the cation exchange membrane, such as accomplishment of the stable electrolysis for a long period under high-current density. Moreover, it has been shown that performance of a cation exchange membrane and allowable current density are largely influenced by, for example, concentrations of catholyte and of anolyte, and pH of anolyte. Accordingly, a necessary requirement resides in the prevention of undesirable effects caused by evolving gas and of local differences in current as well as temperature distributions in the compartments.
Conventionally, countermeasures for equalizing locality in concentration of electrolyte and for preventing adverse effects by gas have been directed to the provision of a circulation system including a pump and a tank to conduct forced circulation between electrolytic cells and the tank. But the conventional measures above are not free from defects of increase in utility cost, equipment costs in tanks, pumps or pipings for circulation. Also, current leakage through the circulation system adversely reduces current efficiency.
Some attempts have been made to overcome the above defects, as shown in the following publications, by producing natural circulation of electrolyte, utilizing gas-lift effect and local difference in gross density of the electrolyte.
(a) Japanese examined patent publication No. 737/65 PA0 (b) Japanese examined patent publication No. 26194/80 PA0 (c) Japanese unexamined utility model publication No. 3958/78
This publication sets up two separate electrolyte routes, gas-contained and gas-free, by combining gas-evolving compartments and gas-free ones, respectively, so as to line up a converged loop. But this attempt requires complex modifications to an electrolytic assembly.
This publication discloses an exterior circulation system comprising upper flowout and lower flowin piping and connection therewith. But this sort of exterior system increases unfavorable current leakage.
This publication attempts to set a guide plate in the electrolyte mass so that upward flow, aroused by gas bubbles, may be separated from downward flow. But this idea requires voluminous compartment space, therefore apparatus efficiency is lowered.
Besides, Japanese unexamined utility model publication Nos. 42027/80 and 42054/80 disclose provision of dispersion nozzles beside a feeding hole for fresh electrolyte, with an idea of equalizing concentration across a compartment. But the dispersion nozzle incurs blocking problems during operation.